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Circe
The Sirens, Scylla and Charybdis.
Circe told him to.
Scylla only ate up six men at a time; Charybdis would wreck the ship by whirlpool.
Odysseus chooses to pass by Scylla because, Scylla will only kill 6 men, one for each head. Charybdis will sink the ship, killing everyone. Circe has also advised Odysseus to do choose Scylla. Further, Charybdis is a visible danger, and Odysseus uses Charybdis to encourage his men to pass by quickly. Finally, passing by Scylla and Charybdis is given as the only way for Odysseus to get home from where he is.
Circe advise Odysseus to sacrifice 6 men by sailing quickly past Scylla and avoiding Charybdis altogether.
Sail on the side of Scylla thereby sacrificing only 6 men instead of the entire ship.
Circe was certainly one, I think at least one of Scylla and Charybdis was female...
Circe warns Odysseus of the dangers in store for him and his crew on their journey home. These dangers include the Sirens, Rovers, Scylla and Charybdis, and Helio if his cattle are slaughtered.
Odysseus chooses to go on Scylla's side of the strait because he believes facing the danger of Scylla is a less risky option than facing Charybdis, the whirlpool monster. He hopes to minimize casualties by sacrificing only a few men to Scylla, rather than risking losing the entire ship to Charybdis.
The Underworld and then through the Harpies and sea monsters; Scylla and Charybdis.
Circe advised Odysseus to sail closer to Scylla and sacrifice a few sailors rather than risk losing his whole crew by getting too close to Charybdis and potentially sinking the entire ship. She warned him that it was better to lose a few men than risk the lives of everyone on board.